1991 Pontiac Firebird Motor Knocking Badly HELP

Hello Phauxy

Water pumps typically fail one of two ways: the shaft seal starts to leak, or the impeller inside breaks, comes loose or the blades erode and wear down (which is more of a problem with pumps that have plastic impellers)
One way to test for a bad water pump is to squeeze the upper radiator hose when the engine is hot and idling. Careful, because the hose will be HOT! If you do not feel much coolant circulating through the hose when you rev the engine, the pump may be bad.
I believe the problem is somewhere else not the water pump.
A rapping or deep knocking engine sound is usually bad news.
A deep rapping noise from the engine is usually "rod knock, " a condition brought on by extreme bearing wear or damage. If the rod bearings are worn or loose enough to make a dull, hammering noise, you're driving on borrowed time. Sooner or later one of the bearings will fail, and when it does one of two things will happen: the bearing will seize and lock up the engine, or it will attempt to seize and break a rod. Either way your engine will suffer major damage and have to be rebuilt or replace.
The engine may be seized due to bearing failure or internal damage (Use socket and long handle to see if it can be turned by hand, if not, engine is locked up.

Hello_
thanks for giving more information, thought engine was dead(killed).
Now tell me, in which circumstance do you hear the knocking noise?
Is this related to engine speed? (Accelerating?)
I understand it is a random knocking but tell me what you as driver are doing when you hear it.

Hello_

Engine noise is due to excessive clearance between two metal parts that should have little to no clearance.
To isolate the origin of the noise, remove the drive belt and start the engine. If the sounds disappears an accessory, accessory bracket, belt or pulley has failed.
Look for signs of rust; this indicates broken or loose metal parts rubbing together, which can generate squeaks and ticking noises. Replace the failed bracket or pulley as needed. If the bracket is ok rotate each accessory by hand to check for hard spots or seized bearings in the accessory and replace as needed
If you still hear the sound, check engine oil, if dirty, replace it with new oil filter.
If you still hear the noise after replacing the oil and filter, the noise is with the valve train.
Too much clearance in valve train could cause that problem.